The invention relates to a device for attenuating electromagnetic cavity interference waves which occur in the vaccum system of a high-frequency electron tube, in particular a transmitter tube.
In high-gain high-frequency electron tubes, cavity resonances result in disturbing cavity waves in the cavity of the electron tube, i.e. so-called interference modes. Transmitter tubes, constructed with coaxially disposed cylindrical electrodes, which are connected to a cavity resonator forming an anode circuit, with the electrodes combined into a single component operating with high operating conductance, are particularly subject to this type of interference. In such type of tube the electromagnetically active, coaxial length of grid and anode is equal to one quarter of the useful wave length, with the corresponding .lambda./4-tuning being effected by means of a coaxial shorting ring disposed between the grid and anode.
FIG. 1 of the drawing schematically illustrates, in axial cross section, such a prior art tube employing cylindrically shaped electrodes, in which the reference numeral 1 designates the cathode, which is surrounded by a grid indicated generally by the reference numeral 2, with the cathode-grid assembly being disposed within a hollow cylindrical anode structure 3, provided with a shorting ring 4 disposed between the grid 2 and the anode 3. The diameter of the grid is designated by the reference letter "d", and the diameter of the anode 3, in the electrically active region of the grid 2, is designated by the reference letter "D". The length of the coaxial portion of grid 2 and anode 3, as defined by the shorting ring 4, is designated by the reference number l', While the axial distance between the terminal of the anode 3 and the grid 2 is designated by ".DELTA.", with l'+.DELTA.=l. The length l' is then approximately .lambda./4, i.e. one quarter of the wave length of the useful frequency, which, for example, maybe 470 MHz and 790 MHz, in European television bands IV/V.
In a coaxial construction with a grid diameter d and an anode diameter D, the critical wave length may be defined by the equation ##EQU1## where the corresponding critical frequency is the lowest frequency of the waves which are propagated on the coaxial conductor formed by the grid 2 and the anode 3, and which may be assumed to be of infinite length, namely the frequency of the H.sub.11 wave.
The axially closed anode 3 and the shorting ring 4 between the grid 2 and anode 3 form a coaxial cavity resonator, in which standing waves can form, which are present as interference modes. In this case the lowest interference frequency is that of the H.sub.11 wave, which forms as H.sub.111 cavity resonance wave having a length l as half wave length. This resonance wave length .lambda..sub.Res possesses the following relationship with respect to the critical wave length .lambda..sub.g and the resonator length l: ##EQU2##
The two formulae (1) and (2) may be employed to calculate the frequency of the H.sub.111 -wave for given dimensions of the electron tube. Other resonances can also occur at higher frequencies. In the absence of special provisions, such resonances are substantially unattenuated, and in electron tubes having high conductance, such as required, particularly for high-frequence transmitter tubes, under suitable feed back conditions self-excitation can take place.
In order to prevent resonances of this type, German OS No. 25 26 127=U.S. Pat. No. 3,970,971 suggests the disposition of attenuating material at those points of the anode at which the interference mode currents flow. This is achieved by embedding ferrite rods into the anode which are disposed transversely to the currents involved. The interference mode currents which are to be attenuated are thereby compelled to flow in loops around the ferrite rods, and are thus magnetically attenuated. However, this method of attenuation, employing ferrite material, involves the disadvantage that such material has characteristics which are detrimental to the vacuum of the tube.